Single bit per pixel graphic image systems, such as those employed by popular personal computers such as the Apple MAC+and MAC SE (tm), are very limited in the types of pictures that can be displayed on the display device, usually a cathode ray tube. One of the techniques that one bit-per-pixel based images may be made to appear to have a gray scale is by "half toning". By half toning is meant the technique of controlling the arrangement and number of black (or white) pixels so as to create the appearance of shading. Another technique is "patterning". By patterning is meant the process of creating a vertical and/or horizontal small area pattern of pixels, and then repeating this stored pattern over and over again in both dimensions.
Computer graphics systems, particularly those of the single-bit-per-pixel variety, typically employ a progressive scan format and typically have a number of scan lines which does not correlate to the number of scan lines of the conventional NTSC interlaced scan format of the commercial broadcast television signal standard. While computer graphics systems typically come equipped with some display device, such as a nine-inch cathode ray tube, many emerging computer graphics applications are facilitated by or actually require scan conversion so that the computer graphics image may be displayed on conventional NTSC interlaced scan display devices.
When a one-bit-per-pixel graphic image is scan-converted into another scan rate, half tones and patterns can be very difficult to represent due to sampling limitations (i.e., Nyquist limit problems). One major problem arising from such scan conversion is flicker which is apparent in the scan-converted resultant image display. Flicker is a particularly severe problem when progressively scanned computer graphics images are converted into an interlaced scan format for display on conventional television signal format image display devices. If the energy at a location in the resultant scan-converted interlaced video image is unequal from field to field, flicker results, the intensity of which being dependent upon the energy differential from field to field.
While scan converters are known in the prior art, one prior solution to the flicker problem is simple vertical low pass filtering to eliminate or sharply curb the appearance of flicker. The apparent drawback of vertical low pass filtering is loss of vertical resolution in the scan converted display. Thus, a hitherto unsolved need has arisen for a scan converter which includes adaptive vertical low pass filtering to remove flicker while optimizing apparent resolution of the scan-converted television image.